I posted the spreadsheet I have been using to look at profitability / ROI on asics.

Download it, enter your gh/s, and then put a 0 in the last column (Bitcoin) on the period which you will receive your asic. By my calcs, at 20% diff increase each period, 200 gh/s worth of October 31st BitFurry will never pay off.... comments welcome.

Difficulty (maybe price..) will adjust so that ROI for a miner is sane.. say 100% ROI between 1 and 3 years.

This only changes when the TH/$ changes... and currently doesn't seem like it will much. Only asicminer can drive it the furthest as they have the cheapest marginal cost to add hash power.

It is growingly rapidly because the ROI is, even still, insanely good for asics. It has been, and still is, SUPPLY contrained. demand is insane as it always has been. When BFL's backorder is shipped, Avalon chips all delivered, asicminer hardware is online and ROI is like 9 months, it will grow much, much more slowly. then jump when KNC delivers, and perhaps when bitfury ships. After that, what? Are people going to be spending thousands that might pay itself back, if everything goes ok, in 18 months? and more likely will be more than that as the network will still go up as AM and some others add hardware

This level of growth will stop if only because people can't afford to keep doing it, and it would be stupid to add hashing power unless prices goes up. Since it's stabilized, and dropped so much to get to this point, it looks like the frenzy should be over after a few months.